Optical evidence for the spin-state disorder in LaCo1-x Rh x O3.

We have measured the infrared reflectivity of single-crystalline samples of LaCo1-x Rh x O3 (x  =  0, 0.05 and 0.10) from 10 to 300 K from 0.05 to 0.15 eV. We find that the optical phonons of the Co-O stretching mode depend on temperature and the Rh content. Analysis with three Lorentz oscillators reveals that the spin state of Co(3+) in LaCo1-x Rh x O3 can be understood in terms of a solid solution of low-spin- and high-spin-state Co(3+) ions, and the substituted Rh ion retains some fraction of the high-spin Co(3+) ions down to low temperature.